01 - Stir the active dry yeast and 1 tbsp of granulated sugar into lukewarm whole milk in a small bowl. Let the mixture rest for 10 minutes until it becomes frothy and activated.
02 - In a large mixing bowl, whisk together the all-purpose flour, unsweetened cocoa powder, remaining granulated sugar, and salt until evenly distributed.
03 - Create a well in the center of the dry mixture. Add the egg, vanilla extract, softened unsalted butter, and activated yeast mixture. Stir with a wooden spoon until a shaggy dough begins to come together.
04 - Knead the dough by hand or with a stand mixer fitted with a dough hook for 8 to 10 minutes until smooth and elastic. During the final 2 minutes of kneading, fold in the chopped dark chocolate until evenly dispersed throughout the dough.
05 - Transfer the dough to a clean, lightly oiled bowl. Cover tightly with plastic wrap or a damp kitchen towel. Allow the dough to rise in a warm, draft-free spot for 1 to 2 hours until it has doubled in volume.
06 - Gently punch down the risen dough to release excess air. Divide into 8 equal portions and shape each into a smooth oval bun. Arrange on a baking tray lined with parchment paper, spacing them at least 2 inches apart.
07 - Cover the shaped buns loosely with a damp towel or oiled plastic wrap. Let them proof for 40 minutes until puffed and nearly doubled in size.
08 - Preheat the oven to 355°F (180°C). Whisk together the egg yolk and 2 tbsp of milk to create an egg wash. Brush the tops of the proofed buns evenly with the mixture for a glossy, golden finish.
09 - Bake on the center rack for 15 to 18 minutes until the buns feel slightly firm to the touch and a wooden skewer inserted into the center comes out clean. Transfer to a wire cooling rack and cool completely before filling.
10 - In a chilled mixing bowl, whip the heavy cream with powdered sugar and vanilla extract using an electric mixer until firm, stable peaks form. Gently fold in the melted and cooled dark chocolate with a spatula until fully incorporated, taking care not to deflate the whipped cream.
11 - Using a serrated knife, slice each cooled bun lengthwise, leaving one side attached as a hinge. Generously pipe or spoon the chocolate whipped cream into the opening. Dust with powdered sugar if desired and serve immediately.